This invention relates generally to cooling tower water control systems and more particularly to control apparatus and a method for automatically and accurately controlling the cyclical blowdown operation of the system irrespective of changes in the conductivity of make-up water introduced into the system during blow-down.
As is known in water cooling tower systems minerals build up in the water due to evaporation as the water is cycled through the cooling tower. Various chemicals are used commercially to keep the minerals from salting out (i.e., scaling) since such action retards heat transfer, thereby decreasing the efficiency of the cooling operation. Moreover corrosion inhibiting chemicals are also introduced into the cooling water to prevent the water from becoming corrosive as it is oxygenated during the cooling operation. In order to maximize the number of cycles cooling water can be circulated before its mineral concentration builds up to excessive levels. It is a common practice to try and maintain the water at a desirable Langelier's saturation index that is relatively high, e.g., +2.5 or low, e.g. -0.5, through the use of chemical additives. Such action maintains the alkaline minerals in solution, thereby precluding corrosion, and without the deleterious effects of scaling.
Each time that the water is cycled through the system the total dissolved solids build up further and hence the conductivity of the water increases. When the conductivity of the water has reached a predetermined maximum (at which scaling could occur even with the use of the scale retardant chemicals in the water) coolant water is dumped from the system and replaced with what is known as "make-up" water. In addition due to the dumping of some of the cooling tower water (a process frequently referred to as "blow-down") additional chemical additives have to be introduced to take the place of the proportion of those chemicals which was lost with the dumped water so that the chemical concentration stays the same.
Heretofore electronic devices have been used to automatically effect "blow-down" when the concentration of the water in the cooling tower reaches a predetermined maximum (called the trip point). To that end, such devices have been designed to automatically actuate a solenoid valve controlling the dumping of water to the drain when a predetermined water concentration is detected. The means for detecting the predetermined concentration normally consists of an electrically sensing conductivity probe located in the cooling water system. The trip point, is normally established by utilizing the Langelier's Saturation Index to calculate the conductivity which would occur after a predetermined number of cycles of concentration. In order to determine the maximum concentration at the predetermined cycles of concentration the conductivity of the input water has to be known.
Morr Control, Inc. of Muskogee, Okla. has sold conductivity controllers like those described heretofore, under the designation "Cyclotron". That device includes a conductivity sensing probe in the cooling tower circulation loop for monitoring the tower water as it is cycled through the system and another similar probe in the "make-up" water input line to the cooling tower. Both probes are connected to an electronic unit or controller for measuring and displaying the conductivity of the water as sensed by the associated probes. The Cyclotron controller also includes adjustable means for establishing the predetermined number of times that the water in the system is allowed to cycle through before the water is dumped. That cycle number is used by the Cyclotron controller to act as a multiplier of the concentration of the make-up water to establish the trip point for the probe sensing the cooling tower water conductivity.
While the Cyclotron device as described above is perfectly suitable for cooling tower systems in which the make-up water remains relatively constant, such a controller system has substantial limitations if the make-up water conductivity changes substantially. For example, if the Cyclotron controller is set up for make-up water to have a maximum conductivity of 60 micromhos and to be cycled five times, the maximum conductivity in micromhos, that is the trip point, will be three hundred at a predetermined Langeliers saturation index. If the input water conductivity increases to three hundred micromhos then the trip point of the system would be the conductivity of the make-up water concentrated through five cycles of operation before blowdown (fifteen hundred micromhos). Before the water reaches that concentration it would more than likely begin scaling. Thus, with prior art conductivity measuring systems, like the Cyclotron, substantial changes in make-up water conductivity cannot be accommodated to maintain automatic control.